here is what you do...
0) install SOUNDFLOWER on your Mac
1) get mainstage up and running and producing sound
2) got to mainstage preferences and select AUDIO button at top of pref's
3) select SOUNDFLOWER (2ch) for AUDIO OUTPUT
4) Click APPLY CHANGES button
5) close pref's
6) open GARAGEBAND
7) go to preferences and select AUDIO/MIDI button at top
8) for AUDIO INPUT, select SOUNDFLOWER (2ch)
9) create a new RECORDING TRACK in garageband (not a MIDI/instrument track)
10) show the info panel for this new track... and click BROWSE panel. Down below on this panel, you will see import source SOUNDFLOWER (2ch) and make SURE MONITOR is ON
you are done...
you should now hear the mainstage instrument in garageband and you can record it.
Give it a try!